Frequency-selective transmission circuit



Feb. 16, 1937. GROSSMAN 2,070,656

FREQUENCY SELECTIVE 'rmmsmssxou cmcum Fiied Now. 27, 1954 NETWORK -5 LOSS INVENTOR A.J.GRO$$MAN A 7'7'ORNEY UNITED STATES FA E Iatented Feb. 16, 1937 FREQUENCY-SELECTIVE TRANSMISSION Q CIRCUIT q Alexander J; firo'ssman, lvlineolal'fi'fY assigii'or to Bell Telephone Laboratories, Incorporated,

New York, N. Y., acorporation of New York n iiifiitat tn November 27, 1934," Serial 1%. 754,923-

- comma. (01.1'78-44) The present inven ion r a es am fi having a flat characteristicover adesired frequency range, andwhere desired, a high degree of selectivity between transmitted and suppressed frequency ranges. 5; Y

'I'heinvention provides an improved manner of constructing multi-stage amplifier circuits which .will give high quality amplification at high gain. In accordance with the invention the interstage circuits of a multi-stage amplifier are networks of the same or different construction from stage to stage to securean overall desired amplifier characteristic. In particular these networks may comprise filters or filter sections which in two successive interstage positions have inversely related characteristic impedances at all frequencies thus avoidingthe possibility of self-oscillation of the stage with which-.they are associated. Moreover, these networks have voltage transmission "characteristics which may difier from one another'but which add together to form a.

. desired resultant over the transmitted frequency being approximately. matched to the tube output impedances' to which their inputs are respectively connected.

A better understanding oi the invention and of its various objects and features will be had from the following detailed description taken in connection with the drawing in which:

Fig.1 is a simple circuit diagramto be referred to in the derivation of certain mathematical expressions; V

Fig. 2 is a schematic'diagrami oiaxmulti-stage amplifier of the invention including interstage networks; v

.Q'Figs. 3, 3Aa'nd 3B relate to one type of net- 'wdrkandinetwork characteristic applicable to the acircuitof- Fig. ,2;..and n Figs. 4, 4A and 4B relate to another, type of network and network characteristic that may be used. z

Referring to the generalized circuit diagram of Fig. 1; the transmission properties of the networkwill be defined in terms of the ratio of the 5 output voltage V to the generator voltage e.

When I there exists free transmission and when M rants (1) It is known from consideringtlie etguitne'rit T of a general four-terminal network that: 35

combining (1) an' d iZ) gives:

e Z]1 cosh 0+R smh 0 (3) than tangent 'dsti-ib the treat ses-eta properties of the Fig. 1 circuit;

In Fig. 2 three amplifier stages are shown comprising amplifier stagesA1, A2 and A3 intercoupledr withi-network's Niiand N22. eAnyYsuitableiinput and output circuits may be assumed forstages V K the voltage transmission from the internal e generator of tube A to the grid of tube A and f V 40. v From this it appears that if works N1 and N2 are inverse to each other, and that 0is the image transfer constant of each, the following relations may be set down:

Let V [I Z the voltage transmission from the internal, e 2 generator of tube A3 to the grid of tube A Then from Equation (3) that is, if

511 11 Z e i 1/R1R2 Sinc e in the theoretical transmitting band of the network the real part of 0 is zero, then which is aconstant indicating uniform or flat transmission in the theoretical transmitting band of.the network.

i 1/ is made equal to unity, free transmission is obtamed. I V

The approach to unity of the quantity need not be close. In fact if the value of this expression lies between 7 andclfi, very good results in flat transmission areobtained: a.

A1 and A3 respectively. If it is assumed that net- I Examples of two types of networks are given in the drawing. Figs. 3Aand 3B show one gen-.

eral type of network, the m-derived half-section filter type (with the parameter m=.6), the network N1 (referred to in Fig. 2) being shown schematically in Fig. 3A and the network N2 in Fig. 3B, or vice versa. The calculated loss-frequency curve for this combination of networks used as in Fig. 2 is given in Fig. 3 in terms of the quantity UK where this is equal to J, fin and f2 being respectively any frequency (the 7 variable), the geometric mean'of the theoretical cut-ofi frequencies and the theoretical upper cut-r 01f frequency. n V

It isapparent from Fig.3 that the transmission is flat, out to UK=.9, with sharp cut-ofi at the band edges. In thisfigure, itwill be understood that the whole band is symmetrical about f I the point UK=0.

A type of network giving less sharpness of cut- "off is shown in Figs. 4A and 4B (for networks N1 and N2 respectively (or vice versa) of Fig. 2).

The calculated loss-frequency characteristic is given in Fig. 4. V

In an actual circuit incorporating the Figs. 4A, 43 type of network in the general circuit of Fig. 2, the following values wereused, Tubes A1 and A2 were WesternElectric No. 101D type and tube A3 was 102D type. L1 was a 66.5 millihenry inductance; L2 was a 3.47 millihenry inductance, G1 was a .000238 microfarad condenser; and C2 was a .00456 microfarad condenser. The dotted line in Fig. 4A means that the series inductance and capacity on the output side were omitted, thisbeing permissible since the filter worked into a half megohm grid resistor.

In general under such circumstances the output series elements of the network may be omitted as is also indicated inFig. 3A. I

In N2, C3 was a .000119 microfarad condenser, 04 was a .002280 microfarad condenser, 1a was .a .l33vmillihenry inductanceand L4 was a 6.94 millihenry inductance. 'binations were each resonant at 40 kilocyclesand rthe LzCz and L4C4 combinations were anti-resonant at that frequency. The band cut-offs were The L101 and LsCs comat 35.22 and 45.32 kilocycles.

Many types of networks can be used which will satisfy the conditions given above, that is, whose input image impedances are approximately equal to the generator impedances over the frequency range of interest and which are designed to operate into substantially open-circuit impedance. By operating on open-circuit on their output side, advantage is taken of a six decibel voltage gain as compared with operating into a load impedance equal to the output impedance.

.One example of a suitable type network would be a constant resistance equalizer of either the bridged-T or lattice form. Use could be made of the general lattice type filter disclosed in United States patent to Bode, No. 1,828,454, dated October 20, 1931. Other types will suggest themselvesin relation to specific requirements in given 1 cases. v

7 While three-element tubes have been shown in Fig.2, any suitable type of tube may be used and itlwillfbeadva'ntageousto use tubes of high input impedance such as screen grid tubes, pentodes, or tubes whose input capacity is small. Any suitable number of amplifier stages may be used, the three stages shown being by way of example. If a large number of stages is used, the idea of using networks whose image impedances are the inverse of each other can be extended to other stages in the general manner indicated.

What is claimed is:

1. An amplifier comprising tandem stages coupled by respective four-terminal networks having desired frequency band transmission characteristics, said networks having input image impedances whose product approximately matches the product of the amplifier output impedances out of which they work and having output image impedances terminating open-circuit, the product of said output image impedances being a constant and proportional to the product of said amplifier impedances which is also a constant.

2. An amplifier comprising tandem stages coupled by respective four-terminal interstage networks which cooperate to give the amplifier a desired frequency-amplitude characteristic, said networks having input image impedances whose product substantially matches the product of the amplifier output impedances out of which they work and being terminated on their output sides in substantially open-circuit impedances.

3. In a multi-stage amplifier, a succession of amplifier elements, impedance networks each connecting the output of one amplifier element to the input of the next amplifier element, said networks each having input image impedances approximately equal to the amplifier element output impedances to which their inputs are connected, and being terminated at their output sides in substantially open-circuit impedances, the product of the output image impedances of said networks being substantially a constant independent of frequency throughout a range of frequencies to be amplified.

ALEXANDER J. GROSSMAN. 

